1. Field of the Invention
The present invention relates to tracking apparatus. More specifically, the present invention relates to a system, a tracker, and related methods for tracking spatial position and orientation of an object.
2. Description of the Related Art
It is often desired to track the position and orientation of an object. For example, in the electronics industry it is often necessary to match surfaces or insert parts in predetermined positions. This is especially significant where a robot or robotic arm is used. In the medical field, it is often necessary to track the position of a medical instrument in order to determine the location of an object within a body. For example, knowledge of the position of a surgical tool during neurosurgery or location of a target such as a tumor while radiation therapy treatment is occurring, have always been critical issues.
Also, in the medical field, recent diagnostic advances such as computerized tomographic (CT) scans, magnetic resonance imaging (MRI) scanning, and positron emission tomographic (PET) scanning have greatly facilitated preoperative diagnosis and surgical or radiation planning. Precision and accuracy of the scanning technologies, however, have not been fully developed in order to utilize such diagnostic advances during treatment to their fullest potential. For example, with respect to radiation therapy, it is assumed that the patient's position and the target's position within the patient will be grossly, or nominally, the same at the time of radiation treatment, as it was at the time the radiation treatment plan was created. If the position of the target is not the same as it was at the time the treatment plan was determined, the dose of radiation may not be delivered to the correct location within the patient's body. Because patients are not always positioned properly on the treatment table of the radiation therapy device, which may be a linear accelerator or a cobalt unit, and because organs of a patient may move within the patient from day to day, the target may not be positioned at the exact location where the radiation therapy plan has assumed it would be located. Various systems and tools have been developed to determine the target position and orientation.
The position of an object or tool is typically defined by three translation parameters (x, y, z) and three rotation parameters (pitch, roll, yaw ) corresponding to six degrees of freedom. The translation parameters (x, y, z) indicate three-dimensional position, e.g. forward and back (y-axis), left and right (x-axis), up and down (z-axis), and three rotation parameters (pitch, roll, yaw) indicate orientation of the tool or object, e.g. rotation about the x-axis (pitch), rotation about the y-axis (roll), and rotation about to the z-axis (yaw). Various systems are available for determining the spatial position and orientation of an object. One such system includes use of a mechanical arm to track the location of a medical tool or probe which can be used to further determine the location of a target. In order to locate the target, the tool or probe can be affixed to the mechanical arm having a known reference position. A computer system tracks the tool or probe while an operator repositions the tool or probe along with the mechanical arm. The geometry of the mechanical arm is known such that movement of the tool or probe in conjunction with the mechanical arm provides the computer system continuous position information regarding the tool or probe. In an invasive procedure, the tool or probe can have a fixed length. Thus, contacting the target with the end of the tool can provide a position location of the target. In a noninvasive procedure, a probe, such as an ultrasound device, can be used to locate both the position and the orientation of the target. Recognized, however, is that the mechanical arm can be cumbersome or difficult for the operator to work with. Additionally, the mechanical arm can be subject to inaccuracies caused by component imperfections due to manufacturing tolerances and mechanical wear and by the effects of gravity on the arm, which to varying degrees depending upon the arm orientation, can act to offset the arm position from that calculated.
Another such system can include either sonic, optical, radio frequency, or even magnetic detectors affixed to the tool or object and active radiating emitters and a computer system or unit. In order to determine the six degrees of freedom of the object or tool, generally, at least three points on the object must be detected. Recognized, however, is that the circuitry involved can be cumbersome or can require modification to the tool or object. For example, generally, wiring from the detectors used to transfer the received signal to a decoder and to the computer system or unit must be affixed on or adjacent to the tool or object. Often, such wiring provides an obstacle to the operator. Also, most detectors typically function by detecting the time, frequency, or amplitude differential between the various detectors in receiving usually at least a pair of external source signals from the emitters in order to determine the spatial position of the tool or object. Thus, the emitter or detector circuitry must, by its nature, be complicated in order to provide for the ability to separately activate each detector.
A similar system can instead include either sonic, optical, or radio frequency emitters affixed to the tool or object and receivers such as sonic, optical, or radio frequency sensors, and a computer system or unit. As described with respect to the use of detectors, in order to determine the six degrees of freedom of the object or tool, at least three points on the object typically must generally be detected. The emitters can be either active or passive. Active emitters, however, are subject to the same wiring interference as that of detectors. Wiring generally supplies encoded signals to each of the emitters which function as markers and which are either activated in sequence or provide sonic, optical, or radio frequency signals on different frequencies. Thus, the emitter or external detector circuitry must therefore, by their nature, be complicated in order to provide for the ability to separately activate each emitter. To reduce the complication and the emitter or external detector circuitry, the emitters can instead function simultaneously emitting the same type of signal. Where the emitters produce such same type signal, however, the emitters are subject to co-emitter interference when the emitters overlap each other with respect to the field of view of the sensors.
Unlike active emitters, passive emitters are generally in the form of a reflector and do not necessarily suffer the same wiring limitations. Passive emitters are becoming the preferred type of emitter as they can be installed on virtually any type of object or tool to provide a relative location of the object or tool or a portion, thereof. Passive emitters supply their signal via active radiating external emitters positioned within view of the passive emitters. The signal from the active emitters is reflected by the passive emitters. The circuitry involved with passive emitters is generally less complicated as they tend to function simultaneously, each emitting or reflecting the same type of signal. Passive emitters, however, are correspondingly also subject to co-emitter interference when the emitters overlap each other with respect to the field of view of the sensors.
Both active and passive emission techniques operate by projecting a geometric representation or extension of the object or tool formed by the emitters onto the field of view of a pair of spaced sensors. Various implementations of sensors have been used, the most popular being the use of two cameras positioned spaced apart a known distance and angled in the general direction of the object or tool such that the three-dimensional position of the object or tool can be obtained by triangulation from the positions of the emitters. For example, a camera or opti-electrical motion measurement system, known as the Polaris®, by Northern Digital Inc., Ontario Canada, has been used for triangulating the position of optically trackable tools.
Specifically, a computer system, using mathematical processing, can determine the three dimensional coordinates of each one of the emitters associated with the object or tool. The position of each of the emitters can be used to determine the position of the object or tool relative to a three dimensional coordinate system centered at a preselected point in space, typically at a point fixed relative to the sensors. The positional relationship to each other of each of the emitters associated with the object or tool can be utilized to further determine the orientation in space of the object or tool. Generally, at least three of the emitters must be detected and must be unobscured by any adjacent emitters. Additionally, the sensors generally require the emitters to be a minimum distance, for example, 3-5 cm apart. Theoretically, such systems should provide three unobstructed emitters for most of a sphere created by the six degrees of freedom. One of the more modern types of passive emission system utilizes passive retro-reflectors which can be affixed to the object or tool and which reflect directly back to a pair of active emitter arrays adjacent a pair of optical sensors. This type of system allows the optical sensors to be positioned relatively close together.
Recognized by the Applicant is that the active systems using a single frequency, wavelength, and amplitude, and the passive systems which inherently do so, are subject to significant field of view limitations. For example, where the three or more emitters are positioned on the tool or object, the emitters will tend to line-up or occlude each other for a large segment of the sphere created by the six degrees of freedom. Where emitter interference occurs, all emitters involved are generally deemed by the computer to be unreliable or unusable. If one or more of those emitters are required by the computer to determine the position of the tool, the tracking of the tool will be lost until three unobstructed emitters are reacquired in the field of view of the sensors. Though in some instances, the prior art has tried to “wallpaper” the tool or an object with upwards of 24 emitters in order to have at least three unobscured emitters, still elusive has been a system, tracker, or related methods for providing at least three unobscured emitters throughout substantially the entire sphere created by the six degrees of freedom.
Also recognized by the Applicant is that mounting the emitters directly to a tool or object frequently exacerbates any existing obstruction problems. For example, if the emitters are mounted directly on the handle of a tool, the operator will have to try to work around the emitters to cover or otherwise obscure them. Additionally recognized by the Applicant is that so as not of interest on a tool or object can be directly determined by the orientation of the emitters. Thus, any mount positioned on the tool or object to carry the emitters must be precisely ed in the correct juxaposition in order to prevent calculation errors. Such mount should also be capable of being easily and quickly disconnected and accurately and repeatably reconnected. For a tool, such as an ultrasound device, which may be frequently separated from the mount in order to clean, service, or inspect the device, it may be imperative to productivity to have such a connection.